Mechanisms regulating metabolism of mitochondrial DNA-encoded mRNA (mtmRNA) for enzyme complexes of oxidative phosphorylation system are not well understood although evidence indicates that alterations in mitochondrial energy metabolism likely to contribute to the pathophysiology of neurodegenerative diseases. We have observed that elevation of intracellular sodium ([Na+]i) markedly decreases levels of mtmRNA in cultured neurons. This was unexpected since high [Na+]i increases energy demand which normally up-regulates mitochondrial metabolism. Long-term Goal: To understand the mechanism(s) of Na+-dependent regulation of mitochondrial gene expression and to determine how this may contribute to selective neuronal vulnerability to delayed neuronal death in stroke and neurodegenerative disorders. Specific Aim: Test the hypothesis that inhibition of mitochondrial gene expression by high [Na+]i is elicited by an excitotoxic insult and that it contributes to delayed neuronal death. Rationale: High [Na+]i following exposure of primary neuronal cultures to excitotoxic levels of glutamate inhibits mitochondrial gene expression, compromises cellular energy metabolism, affects electron transport-dependent mitochondrial activities (e.g., ATP synthesis, [Ca2+]i buffering free radical generation and sensitivity to Ca2+-induced release of cytochrome c: and increases neuronal vulnerability to both necrotic and apoptotic cell death. Approach: Levels of mitochondrial mRNA, respective protein subunits, associated enzyme activity (i.e., cytochrome oxidase activity) and mitochondrial respiration will be monitored in primary neuronal cultures after exposure to excitotoxic levels of glutamate and compared to cellular ATP levels and markers of neuronal death over 24 hours in the absence and presence of a delayed, secondary glutamate exposure. Significance: This project examines a new mechanism by which elevated [Na+]i contribute to delayed cell death. An understanding of this mechanism could lead to the development of novel neuroprotective interventions.